Objective: To determine extracellular calcium (Ca(2+)) requirements for the maintenance of human sperm function in vitro.
Design: Prospective study.
Setting: Basic research laboratory.
Patient(s): Normozoospermic volunteers provided fresh semen samples; follicular fluid (human FF) and oocytes were collected from women undergoing IVF-ET.
Intervention(s): Spermatozoa were incubated for </=18 hours in media containing different CaCl(2) concentrations (maximum, 2.5 mM [control]).
Main outcome measures: Protein tyrosine phosphorylation patterns, development of hyperactivated motility, induction of the acrosome reaction (AR) in response to human FF, and sperm interaction with homologous zona pellucida (ZP).
Result(s): Cells maintained for 18 hours in medium containing >/=0.1 mM of Ca(2+) were able to undergo the AR when exposed to human FF in the presence of 2.5 mM of Ca(2+). Calcium concentrations of >/=0.22 mM were sufficient to reach protein tyrosine phosphorylation levels and hyperactivated motility values similar to those of controls. Higher Ca(2+) concentrations (>/=0.58 mM) were required to produce maximum human FF-induced AR in previously capacitated cells and to obtain an adequate sperm-ZP binding.
Conclusion(s): Different steps of the fertilization process have distinctive Ca(2+) requirements. Whereas 0.22 mM of Ca(2+) is sufficient for the development of some capacitation-related events, human FF-induced AR and sperm-ZP interaction require 0.58 mM of this cation.